Numerous constructions of oscillator-operated, proximity detectors, which emit a signal only when the target object is at a distance greater than a predetermined value, are known. The present invention relates to a d.c. supplied proximity detector of the type having only two wires for connecting the supply source and the load circuit to the proximity detector, the latter being capable of emitting a signal as well when the target object is at a distance lower than a predetermined value, as when the target object is at a distance greater than the said predetermined value. This device has three terminals, the first being directly connected to one pole of the d.c. supply source, whilst the second and third terminals are connected to the other pole of the source through the load circuit. This detector comprises in per se known manner a transistorised oscillator associated with a detector of the oscillating generated by the oscillator, a voltage stabiliser comprising a transistor in series with a zener diode and static switching members.
The invention aims at permitting the said proximity detector to withstand large supply voltage variations, to suffer no damage in the case of erroneous polarities in the connection of components, and to ensure in all cases a clearly defined operation.
The device detects the close presence of a metal object by damping of the oscillation of its oscillator, and the second terminal operates as a closed contact when the object is not in close proximity, whereas the third terminal operates as a closed contact when the target object is in close proximity. This corresponds to safety operation because proximity cannot be confused with a failure in the power supply.
According to the invention, the said proximity detector is a static switching device comprising first and second transistors, the conductive state of one or other of the said transistors being controlled by the output state of an inverter in turn controlled by a detector, the two transistors being connected to a first terminal via a zener diode, second and third terminals being interconnected by means of two serially connected diodes of opposed polarities whose common point supplies, voltage stabiliser including a Zener diode the current flowing through the zener diode of the said stabiliser being determined by two resistors, one of which is connected to the second terminal and the other to the third terminal.